A heat dissipation module is necessary to provide effective heat dissipation for a portable electrical apparatus, and especially for a notebook personal computer (PC), so as to allow the best performance. When heat accumulates and cannot dissipate in the apparatus, the electrical devices and the apparatus as a whole cannot work normally. Seriously, the computer system, in particular, will crash. However, in order to satisfy the market requirement for mobilization, sufficient space has not been left in the casing of the notebook PC for natural convection. Because heat dissipation design for high-frequency devices is at a bottleneck, the heat dissipation module of the notebook PC tends to dissipate heat with forced convection generated by a centrifugal fan.
Reference is made to FIG. 1, which depicts a three-dimensional structure showing a centrifugal fan of the prior art. The centrifugal fan 10, installed in a portable electrical apparatus such as a notebook PC, comprises a casing 11 and blades 31. The casing 11 has a volute sidewall 13 and a bottom plate 15, and the casing 11 defines a cavity 17 and an outlet 19. A volute channel 21 is formed from the cavity 17 and the outlet 19, and a tongue shape 23 is formed from the volute sidewall 13 on a side of the outlet 19. The elongate axis of each blade 31 is vertically with a rotational axis 33 on the bottom plate 15 in the cavity 17, and the surface of each blade 31 is parallel to the rotational axis 33. Rotation of blades 31 drives a horizontal airflow 25. The horizontal airflow 25 can be exhausted via dynamic and static pressures generated by the horizontal airflow 25 passing through the volute channel 21. The outlet 19 of the prior art uses a curved surface and divergent duct with a constant section, which means a surface 16 of the volute sidewall 13 at the tongue shape 23 is parallel to the rotational axis 33; in other words, an angle of the surface 16 is 90 degrees relative to the bottom plate 15. This makes the air flow generate dynamic and static pressure, and the air resists outside force to achieve the effect of heat dissipation.
Reference is made to FIG. 2, which depicts a schematic diagram showing a wake flow generated by the centrifugal fan of the prior art blowing on the volute sidewall at the tongue shape. When the wake flow 35 generated by the blades 31 is blowing on the surface 16 of the volute sidewall 13 at the tongue shape 23, each blasting point 41 is generated and regarded as a sound source that generates narrow band noise 43. Because the surface 16 of the volute sidewall 13 at the tongue shape 23 is parallel to the rotational axis 33, according to the superposition principle, the narrow band noise 43 with large amplitude 45 and constant blade pass frequency will be generated. Once it is necessary to enhance the efficiency of the heat dissipation, the flow rate and the wind velocity must be increased. However, the more the rotational speed and the flow rate are increased, the higher the frequency of the noise band is. Therefore, the noise is increasing.